WATCH: Speaking outside her offices in London, British Prime Minister Theresa May says that it is "beyond doubt" that Britain and the city of Manchester have fallen victim to "a callous terrorist attack."

Authorities confirmed that the incident was a suicide attack and they believe it was carried out by one man, Manchester Chief Constable Ian Hopkins said Monday night.

“The priority is to establish whether he was acting alone or as part of a network,” he said.

“The attacker, I can confirm, died at the arena. We believe the attacker was carrying an improvised explosion device which he detonated causing this atrocity.”
Video coverage of Manchester Arena explosion

The explosion was reported at 10:33 p.m. local time as the concert drew to a close, Hopkins said. The exact location of the reported blast is not clear, with a British Transport Police statement suggesting it happened in the arena’s foyer area, and the Manchester Arena tweeting that the incident “took place outside the venue in a public space.”

The injured are being treated at six hospitals across Greater Manchester, Hopkins said, adding that people are urged to avoid the area to enable investigators and response personnel to work unhindered.

On Tuesday, police confirmed an arrest was made in connection to the attack. Greater Manchester Police said a 23-year-old man was arrested in South Manchester just before noon local time.

With regards to last night’s incident at the Manchester arena, we can confirm we have arrested a 23-year-old man in South Manchester.

Police earlier carried out a precautionary controlled explosion in the area after a suspicious item was found near the site of the reported blast. The item was later confirmed to be abandoned clothing rather than a suspect device.

WATCH: Concertgoers describe moment of blast as panic quickly set in at arena in Manchester, England

The North West Ambulance Service tweeted that area residents requiring ambulance services may face delays because of the large number of resources directed to the arena.

Prime Minister Justin Trudeau issued a statement on Monday saying that authorities are “still in the process of determining whether any Canadians have been impacted by this event.”

“With our allies and partners, we will continue to counter such senseless acts and to fight terrorism in its many forms,” he said. “We will do all that we can to assist British authorities in bringing those responsible to justice.”

Canadians are shocked by the news of the horrific attack in Manchester tonight. Please keep the victims & their families in your thoughts.

Witness Andy Holey told the BBC that an explosion sent him flying about 30 feet, as he waited at the arena to pick up his wife and daughter, who were attending the show.

“When I got up I saw bodies lying on the ground… it was definitely an explosion and it was some force. It happened near the box office at the entrance to the Arena,” Holey said, adding that his family was found safe.

WATCH: Video captures sound of explosion outside Manchester Arena

Another witness told Reuters she felt a massive explosion as she was leaving the concert.

“We were making our way out and when we were right by the door there was a massive explosion and everybody was screaming,” Catherine Macfarlane said. “It was a huge explosion – you could feel it in your chest. It was chaotic. Everybody was running and screaming and just trying to get out of the area.”

Police and other emergency services are seen near the Manchester Arena after the reported explosion, May 22, 2017.

Joel Goodman/LNP/REX/Shutterstock (8828037b)

Police and other emergency services are seen near the Manchester Arena after the reported explosion, May 22, 2017.

Joel Goodman/LNP/REX/Shutterstock (8828037c)

Police and other emergency services are seen near the Manchester Arena after the reported explosion, May 22, 2017.

Emergency services personnel speak to people outside Manchester Arena after the reported explosion, May 22, 2017.

Peter Byrne/PA via AP

Ariana Grande was in the midst of a European tour, with two concerts scheduled for London later this week.

Chris Pizzello/Invision/AP, File

There was widespread confusion and panic in the immediate aftermath of the explosion, with some people screaming that they’d seen blood but others saying it was nothing more than a popped speaker or balloons bursting, the BBC reported.

Greater Manchester Police said they are treating the explosion as a terrorist incident until they learn otherwise. British Prime Minister Theresa May said authorities are “working to establish the full details of what is being treated by the police as an appalling terrorist attack.”

If confirmed as a terrorism incident, this would be the deadliest attack in Britain by militants since four British Muslims killed 52 people in suicide bombings on London’s transport system in July 2005.

Campaigning has been suspended in Britain’s election before citizens head to the polls on June 8.

Prime Minister Theresa May is to chair a meeting of COBRA, the British government’s emergency committee, after cancelling campaign events on Tuesday.

Britain is on its second-highest alert level of “severe,” meaning an attack by militants is considered highly likely. In March, four people were killed and 50 injured in a terrorist attack outside the U.K. Parliament.

The Manchester Arena opened in 1995 and has a capacity of 21,000, according to the venue’s website. It is one of the busiest entertainment venues in the world, and the biggest indoor arena in Europe.